Stanisław of Skarbimierz

"Do not pick a quarrel with a man for no reason, if he has not done you a bad turn," counseled King Solomon (Proverbs 3:30). Even more so, do not start a war against the entire nation, kingdom, principlity or society that did you no wrong. Jesus told us, "All who take the sword die by the sword (Matt. 26:52)."

This has been said about
those who take up arms without an explicit order from a monarch or a judge.
As St. Augustine says in Book 2 of his treatise against the Manicheans
(Contra epistolam Manichaei quam dicumnt fundamenti), "taking
the sword" means taking up arms without the order or permission from
the authorities. If the monarch or the judge commands us to take up arms,
then the sword is used because someone else ordered us to use it, and
the user is not to be punished. those who ordeed it, if the cause is unjust,
will be punished. . . .

Thus kings, princes, counts,
and barons should diligently consider the question of whether the wars
they wage are just or unjust. The Psalmist says: "Be mindful then,
you kings; learn your lesson, rulers of the earth:worship the Lord with
reverence (Ps. 2:10-11)." This also means: before you begin a war,
found out whether it will be just or unjust, so that you know whether
justice is on your side.

Therefore, no king or prince is allowed to rise against another kingdom or principality without a just reason; such actions are forbidden by God and the law. If the king or ruler does not obey, he can look forward to the most severe punishment, and the greater the damage he caused and the larger the number of people he killed, the more severe the punishment will be.

In some cases, a person
can take the sword to regain stolen property or to defend his homeland
without an order of a king or the Church; such is the opinion of Brother
Wilhelm from Rennes and Pope Innocent IV. Of course one has to pay attention
to the circumstances. But if a king or a monarch attacks someone's country,
occupies its cities and steals its wealth and if the aggrieved party cannot
instantly produce an army to take the cities or the wealth back, but does
so only after a period of time, say a few months - that party is entitled
to take back whatever was taken away from it.